Up until earlier today I crossed it at 80hz, then started experimenting and now it's crossed at 35hz, 12db slope, -8dB on the elf trim. To be honest it sounded better at 80hz.As a C1 owner with a Rel B3 I found the integration to be somewhat time consuming. First I want the sub to pick up where the C1's drop off - not add to it. For starters I would say you need to get the timing/phasing just right. You don't want the sub to lead or lag the mains. Since my Rel's phase switch is only 0 or 180 degrees I had to physically move the sub closer or farther to get it to blend in. I would also start with just 1 sub and get it right (because all Dyn's do sound great all by themselves but do need a little help in the lowest of octaves). Then work on the gain. Once it is integrated properly you shouldn't be able to hear where the sub is. But the hard part here is what is right. Most recordings don't go that low and if you increase the gain to compensate for what is not there you will have too much for the recordings that do go that low. At least that is what I found. Now add the second sub and repeat the process (starting with the timing/phasing) making sure sub #2 doesn't cancel out sub #1. Then go back and adjust the gain.
A second option would be to get an external crossover and cut the 160's off at 80hz and have the subs pick it up from there. Again the timing/phasing still has to be right to sound good.
That being said if you go the 380 or C2 MKII you will probably want to upgrade the Sim 340i.
Best of luck what ever you decide.